1. Field of Invention
The present invention relates to a system and method for delivering chemicals to a tool and, more particularly, to a chemical delivery system and method utilizing at least two sources of the chemical.
2. Discussion of Related Art
Chemical delivery systems are used, for example, in the semiconductor pharmaceutical, and cosmetic industries. Semiconductor manufacturing typically utilizes chemical distribution systems to deliver chemicals to a process tool. In particular, slurry distribution systems deliver a slurry for chemical mechanical polishing (CMP). Oftentimes, it is desirable to provide a precise flow rate of the chemical or slurry to the process tool. Flow meters which may be susceptible to changes in input pressure are commonly used to deliver slurries and chemicals to the tool.
Chemical delivery systems commonly include duplicate sources of chemical, in order to avoid down time. Changes in input pressure to the process tool may occur when switching delivery of a chemical from one supply source, typically a mixing tank, to another. Even minor fluctuations in process parameters associated with such transitioning, however, may lead to a significant disruption of the continuous delivery of the chemical and/or quality of the process or product. When an in-service mixing tank goes off-line, there may also be low or residual chemical present in the tank which may cause a pressure drop in the supply line to the process tool. In addition, residual chemical in the tank typically represents waste with an associated cost. In addition to the loss of residual chemical in the tank, there may be significant dead leg loss with chemicals remaining in now off-line process piping.
U.S. Pat. No. 7,007,822 to Forshey, et al, discloses a chemical mix and delivery system. In Forshey, a mixing tank is also a main reservoir of chemical to be delivered to a tool. One or more buffer reservoirs are positioned downstream of the main reservoir for delivery to a tool. A programmable loop controller controls the pressure in each buffer reservoir to achieve a desired flow rate of CMP slurry from the buffer reservoirs. To clean and/or flush the main reservoir, the controller interrupts flow to the buffer reservoirs while DI water is added to the main reservoir and sent to a drain. The process tool determines from which of two buffer reservoirs the chemical slurry will be drawn.